Because the 14-volt electrical system in present automobiles has reached its limits of capability, a 42-volt power network has been proposed to cope with the increasing electrical loads. This is particularly true for electrical and hybrid vehicles in which more engine-driven mechanical and hydraulic systems are being replaced with electrical systems to increase efficiency. These vehicles also utilize at least one high voltage bus exceeding 100 dc volts. During the transition to the 42-volt system, most automobiles are expected to employ a dual-level voltage system, in which a bi-directional dc-to-dc converter will interface the two voltage networks. Additionally, battery-internal combustion (I.C.) engine hybrid electric vehicles (HEVs) or fuel-cell-powered electric vehicles (FCEVs) employ a third dc bus at a much higher voltage (typically above 200V) (H.V.) for the traction motor drive. In these HEVs and FCEVs, the dc-to-dc converter controls power on the 14-volt, 42-volt and high voltage buses.
In a prior U.S. Pat. No. 6,370,050, assigned to the present assignee, a dc-to-dc converter for interfacing these buses was disclosed. Although not shown in this patent, a large filter capacitor (several hundreds to thousands of microfarad) was needed to connect a 14-volt bus in parallel with the loads to maintain a reasonably smooth dc voltage at the bus. And yet, the dc voltage still fluctuated on the low voltage side, as the switches S1 and S2 turned on and off, because the voltage at the midpoint between the two switches rapidly changes between the ground potential and the 42-volt bus. A fluctuating band is determined by the switching frequency of the two power semiconductor switches, S1 and S2, by the capacitance of the large filter capacitor (not shown) and by the inductance Ldc of a large inductor in the 14-volt bus. A high switching frequency and/or a large inductor or filter capacitor is needed to keep the voltage fluctuations small, which increases power losses in the switches and inductor and/or the size and volume of the inductor and filter capacitor. In addition, the lower switch S2 needs to carry a much larger current than that passes through the upper switch S1 because in addition to the current from the transformer, Tr, the current of the inductor flows through the switch when it is turned on.
A dc-to-dc of similar architecture was disclosed in Su, Gui-Jia and Peng, Fang Z., “A Low Cost, Triple-Voltage Bus DC/DC Converter for Automotive Applications,” IEEE Applied Power Electronics Conference and Exposition (APEC), vol. 2, pp. 1015-1021, Mar. 6-10, 2005, Austin, Tex. In addition to the power semiconductor switches, S1 and S2, on the low voltage side of the converter, there are also typically two additional power semiconductor switches, S3 and S4, on the high voltage side. The above publication discloses an advantageous method of operating the switches using power flow control. The gating signals for the switches can be based on controlling the duty cycle and the phase angle of switching between on and off states.
It is now desired to improve a dc-to-dc converter circuit by providing a circuit with reduced cost, reduced size and reduced power losses. It is also desired to reduce ripple currents (ac currents charging and discharging a capacitor with a dc biased voltage) that are produced by switching of the switches and follow through the dc bus capacitors (C1-C4) in the circuit. Because a capacitor can only handle a fixed amount of ripple current, a higher ripple current requires additional capacitance to bear it; increasing volume and cost.